Method for fabricating carbon nanotube aluminum foil electrode

ABSTRACT

A method for fabricating a carbon nanotube aluminum foil electrode is used to increase the surface area of the electrode by growing the high-electric conductivity carbon nanotubes on an aluminum foil or an AAO (Anodic Aluminum Oxide)-containing aluminum template, whereby is greatly increased the capacitance of the aluminum electrolytic capacitor. The electrode fabricated via forming the carbon nanotubes on the AAO-containing aluminum template has a capacitance of 2158.6 μF/cm 2  and an operation voltage of 55.10 V. The electrode fabricated via directly forming the carbon nanotubes on the aluminum foil has a capacitance of 3425.78 μF/cm 2  and an operation voltage of 38.44 V. Therefore, the capacitance of the aluminum electrolytic capacitor is effectively increased. Further, the carbon nanotube aluminum foil electrode fabricated in different ways can meet the different requirements of operation voltages and capacitances.

FIELD OF THE INVENTION

The present invention relates to a method for a capacitor electrode, particularly to a method for fabricating a carbon nanotube aluminum foil electrode.

BACKGROUND OF THE INVENTION

Increasing the capacitance and miniaturizing the aluminum electrolytic capacitor thereof is a critical task in the related field. There are three ways to increase the capacitance of an aluminum electrolytic capacitor: (1) decreasing the spacing between the electrodes, (2) using a high dielectric constant material as the insulating material between the electrodes, and (3) increasing the surface of electrodes. An aluminum-oxide film is formed under a specified voltage, and the thickness of the aluminum-oxide film is proportionally increased by a ratio of 1.3 nm/V. Further, the electrodes have a given spacing therebetween, and the aluminum-oxide film has a fixed dielectric constant permittivity. Therefore, increasing the surface of electrodes is a practicable method to increase the capacitance of the aluminum electrolytic capacitor. The capacitance of the aluminum electrolytic capacitor can be increased via increasing the surface of the anode aluminum foil or the cathode aluminum foil, wherein the anode aluminum foil is anodized to have a thick and dense aluminum oxide film. Further, the cathode aluminum foil has a capacitance much higher than the anode aluminum foil. Therefore, increasing the capacitance of the anode aluminum foil is the most effective method to increase the capacitance of the aluminum electrolytic capacitor.

The surface area of an aluminum foil can be increased with a traditional chemical or electrochemical etching method, wherein an acidic solution that is corrosive to aluminum is used to form rugged pits on the surface. Jinwook Kang, Yunho Shin and Yongsug Tak disclosed a reference “Growth of etch pits formed during sonoelectrochemical etching of aluminum” in Electrochimica Acta, Volume 51, Issue 5, 10 Nov. 2005, Pages 1012-1016. In the conventional technology, different ultrasonic frequencies are used to improve the etch-inhibiting ability of aluminum chloride in an electrochemical etching process undertaken in an electrochemical etching solution (1 M HCl+3 M H₂SO₄) and at a current density of 0.03 A/cm². Then, the forming is processed in a boric acid solution.

The conventional technology can achieve a capacitance of about 2 μF/cm² when the anodizing voltage is 50 V.

Du, et al. disclosed a reference “Formation of Al₂O₃—BaTiO₃ composite thin film to increase the specific capacitance of aluminum electrolytic capacitor” in Thin Solid Films, Volume 516, Issue 23, 1 Oct. 2008, Pages 8436-8440, wherein BaTiO₃ is fabricated by a sol-gel method and coated on an etched aluminum foil. Next, the aluminum foil with BaTiO₃ is heat-treated at different temperatures. Then, the aluminum foil is anodized with an ammonium adipate solution to increase the capacitance. The conventional technology can achieve the capacitance of about 95.66 μF/cm² when the heat treatment is undertaken at a temperature of 600° C.

Nogami, et al. disclosed another reference “The effects of Hyperbranched poly(siloxysilane)s on Conductive Polymer Aluminum Solid Electrolytic Capacitors” in Journal of Power Sources, Volume 166, Issue 2, 15 Apr. 2007, Pages 584-589, wherein hyperbranched poly(siloxysilane)s that contains a lot of vinyl groups is used to improve the interface character between aluminum oxide and the corresponding [poly-(3,4-ethylenedioxythiophene)] electrode. The conventional technology can achieve a capacitance of about 215.79 μF/cm².

A Taiwan patent publication No. 200828368 disclosed an “Electrolytic Capacitor Electrode”, wherein metal oxide is formed on the surface of an aluminum substrate via chemical bonding, and the metal oxide is heat-treated at a temperature of 100-500° C. The capacitance of the metal oxide-coated aluminum substrate can be effectively increased. The metal oxide is generated by the chemical reaction between the precursor thereof and the functional groups on the surface of the aluminum substrate. Therefore, the metal oxide is hard to peel off. However, the conventional technology can only achieve an electrostatic capacity of about 252-263 μF/cm².

SUMMARY OF THE INVENTION

The primary object of the present invention is to increase the surface area of an aluminum foil electrode, whereby the capacitance of an aluminum electrolytic capacitor can be effectively increased.

To achieve the abovementioned object, the present invention proposes a method for fabricating a carbon nanotube aluminum foil electrode, which comprises steps:

Step S1: pre-processing an aluminum foil to remove the grease and oxide layer on the surface thereof;

Step S2: electroplating a catalyst, wherein a catalytic material is electroplated on the aluminum foil;

Step S3: growing carbon nanotubes, wherein the carbon nanotubes are grown on the catalyst-coated aluminum foil;

Step S4: sputtering aluminum on the carbon nanotube aluminum foil, wherein an aluminum target and a radio frequency power source are used to sputter aluminum on the aluminum foil where the carbon nanotubes have been grown, and wherein the sputtering is undertaken with a power of 50-95 W for 2-4 hours; and

Step S5: forming an aluminum oxide layer on the carbon nanotube aluminum foil after sputtering aluminum, the forming step is in an anodizing solution, and wherein the forming is undertaken at a temperature of 83-90° C. for 10 minutes, whereby is completed the carbon nanotube aluminum foil electrode of the present invention.

The present invention is used to increase the surface area of electrodes via growing carbon nanotubes on the aluminum foil, whereby the capacitance of the aluminum electrolytic capacitor is greatly increased. From experiments, it is known that the aluminum electrolytic capacitor fabricated according to the present invention has a capacitance of as high as 3425.78 μF/cm² and an operation voltage of as high as 38.44 V, which are much higher than the conventional technology. Therefore, the present invention can effectively promote the capacitance of the aluminum electrolytic capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a method for fabricating a carbon nanotube aluminum foil electrode according to one embodiment of the present invention;

FIG. 2A is a schematic view of the surface of the pre-processed aluminum foil according to one embodiment of the present invention;

FIG. 2B is a schematic view of the surface of the catalyst-electroplated aluminum foil according to one embodiment of the present invention;

FIG. 3A is a schematic view of the surface of the AAO layer on the aluminum foil according to one embodiment of the present invention; and

FIG. 3B is a schematic view of the section of the AAO layer on the aluminum foil according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Refer to FIG. 1 a flowchart of a method for fabricating a carbon nanotube aluminum foil electrode according to one embodiment of the present invention. The method of the present invention comprises the following steps:

Step S1: pre-processing an aluminum foil: An aluminum foil is pre-processed to remove the grease and oxide layer on the surface of the aluminum foil. In one embodiment, the aluminum foil is soaked in acetone to remove the grease on the surface. Next, the aluminum foil is soaked in a 1 M solution of sodium hydroxide for two minutes to remove the oxide layer on the surface. Next, the aluminum foil is flushed with deionized water. Then, the aluminum foil is soaked in alcohol and ultrasonically oscillated for 15 minutes. Refer to FIG. 2A for a microscopic photograph of the surface of the pre-processed aluminum foil.

Step S2: electroplating a catalyst: A catalytic material is deposited on the surface of the aluminum foil via electroplating. The catalytic material may be cobalt. The electroplating is undertaken in an electroplating solution containing 5 wt. % CoSO₄.7H₂O and 2 wt. % H₃BO₃, at an alternating voltage of 13.6 V and a frequency of 60 Hz, for 40 seconds. Refer to FIG. 2B for a microscopic photograph of the surface of the catalyst-electroplated aluminum foil.

Step S3: growing carbon nanotubes: Carbon nanotubes are grown on the surface of the catalyst-deposited aluminum foil with a CVD (Chemical Vapor Deposition) and via the catalytic reaction of the catalyst, wherein argon is the carrier gas and acetylene is the carbon source. The growth is undertaken at a temperature of 575-610° C. for 15-90 minutes, and the flow rate of argon is 100 sccm, and the flow rate of acetylene is 50 sccm. Sccm is a unit of flow rate, meaning the number of cubic centers of the gas flowing through a point at the standard state (at a temperature of 273 K and a pressure of 760 Torr) per minute.

Step S4: sputtering aluminum on the aluminum foil where the carbon nanotubes have been grown: An aluminum target and a radio frequency power source are used to sputter aluminum on the carbon nanotube aluminum foil at an argon flow rate of 25 sccm, a sputtering pressure of 20 mTorr and a sputtering power of 50-95 W for 2-4 hours.

Step S5: forming an aluminum oxide layer on the carbon nanotube aluminum foil after sputtering aluminum: The aluminum foil is placed in an anodizing solution to form an aluminum oxide layer. The forming is undertaken at a temperature of 83-90° C. for 10 minutes. In one embodiment, the anodizing solution is formed via mixing 150 g of ammonium adipate with 1 liter of deionized water.

The present invention uses the abovementioned steps to form a carbon nanotube aluminum foil electrode. Further, an anodic aluminum oxide layer may be formed after Step S1 via the following steps:

Step S1A: electropolishing the aluminum foil: The aluminum foil is placed in an electropolishing solution and electropolished at a voltage of 30 V and a temperature of 25° C. for 15 minutes. The electropolishing solution contains sulfuric acid, phosphoric acid and deionized water by a ratio of 2:2:3.

Step S1B: performing a first anodizing process on the electropolished aluminum foil: The electropolished aluminum foil is placed in a 0.3 M solution of oxalic acid and anodized at a voltage of 30-40 V and a temperature of 5-15° C. for 12 minutes to form an AAO (Anodic Aluminum Oxide) layer.

Step S1C: performing a second anodizing process on the aluminum foil: In order to make the AAO layer arranged orderly, an oxide-removing solution is used initially to remove the anodic aluminum oxide formed in the first anodizing process, wherein the oxide-removing solution contains 1.8 wt. % chromic acid and 6 wt. % phosphoric acid. Next it is undertaken the second anodizing process. Refer to FIG. 3A. The aluminum foil is placed in the oxalic acid solution and anodized to form a new AAO layer 10 on the imprint left by the first anodizing process. The new AAO layer 10 has a plurality of pores 11. Similarly to the first anodizing process, the second anodizing process is undertaken in a 0.3 M solution of oxalic acid, at a voltage of 30-40 V and a temperature of 5-15° C. for 12 minutes.

Step S1D: removing a barrier layer: Refer to FIG. 3B a sectional view of the AAO layer according to one embodiment of the present invention. After the second anodizing process, the aluminum foil is soaked in a phosphoric acid solution containing 5 wt. % phosphoric acid at a temperature of 30° C. for 40 minutes to remove a barrier layer on the bottoms of the pores 11 to make the pores 11 interconnect the aluminum foil, whereby the catalytic material 20 can be electroplated on the bottoms of the pores 11 to contact the aluminum foil in abovementioned Step S2. Next, following Step S3, the carbon nanotubes are grown along the pores 11 with the catalytic material 20. The AAO layer 10 limits the area where the catalytic material 20 is deposited. However, the pores 11 make the carbon nanotubes hard to peel off from the aluminum foil. Therefore, the carbon nanotube AAO-containing aluminum foil electrode has a higher operation voltage.

The carbon nanotubes are preferably grown at a temperature of 600° C. for 60 minutes for both the carbon nanotube aluminum foil electrode and the carbon nanotube AAO-containing aluminum foil electrode. A higher temperature favors the growth of carbon nanotubes. However, the melting point of aluminum is about 620° C. Thus, the growth of the carbon nanotubes becomes unstable when the temperature approaches 620° C. Experiments show that the carbon nanotubes have the maximum average surface area density with a higher capacitance after they have been grown for 60 minutes. Therefore, the time of growing the carbon nanotubes are preferred to be 60 minutes. In Step S4, the thickness of the sputter-deposited aluminum is increased with the sputtering time and the sputtering power. The thicker the sputter-deposited aluminum, the thicker the aluminum oxide obtained in the forming step. According to the equation of capacity, the thicker the aluminum oxide has a lower capacitance. However, the thicker the aluminum oxide has a higher operation voltage. Considering the abovementioned factors, the sputtering power and the sputtering time are respectively preferred to be 65 W and 4 hours.

The carbon nanotube AAO-containing aluminum foil electrode fabricated according to the abovementioned conditions has a capacitance of 2158.8 μF/cm² and an operation voltage of 55.10 V. But the carbon nanotube aluminum foil electrode fabricated according to the abovementioned conditions has a capacitance of 3425.78 μF/cm² and an operation voltage of 38.44 V. The AAO layer 10 decreases the area where the catalytic material can be electroplated, so that the carbon nanotube AAO-containing aluminum foil electrode has a smaller capacitance and a greater operation voltage. Both the carbon nanotube aluminum foil electrode and the carbon nanotube AAO-containing aluminum foil electrode have a capacitance much greater than the conventional technology. Therefore, the present invention is proved to meet the conditions of a patent. Thus, the Inventor files the application for a patent. It is appreciated if the patent is approved fast.

The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the technical contents or spirit of the present invention is to be also included within the scope of the present invention. 

1. A method for fabricating a carbon nanotube aluminum foil electrode comprising steps: Step S1: pre-processing an aluminum foil to remove the grease and oxide layer on a surface thereof; Step S2: electroplating a catalytic material on the aluminum foil; Step S3: growing carbon nanotubes on the catalytic material-coated aluminum foil; Step S4: sputtering aluminum on the carbon nanotube aluminum foil, wherein an aluminum target and a radio frequency power source are used to sputter aluminum on the aluminum foil where the carbon nanotubes have been grown, and wherein the sputtering is undertaken with a power of 50-95 W for 2-4 hours; and Step S5: forming an aluminum oxide layer on the carbon nanotube aluminum foil after sputtering aluminum, the forming step is in an anodizing solution, and wherein the forming is undertaken at a temperature of 83-90° C. for 10 minutes, whereby is completed the carbon nanotube aluminum foil electrode.
 2. The method for fabricating a carbon nanotube aluminum foil electrode according to claim 1, wherein after the Step S1 comprises a Step S1A: electropolishing the aluminum foil which is placed in an electropolishing solution containing sulfuric acid, phosphoric acid and deionized water by a ratio of 2:2:3.
 3. The method for fabricating a carbon nanotube aluminum foil electrode according to claim 2, wherein after the Step S1A comprises a Step S1B: performing a first anodizing process on the aluminum foil, wherein the electropolished aluminum foil is anodized in an oxalic acid solution to form an Anodic Aluminum Oxide layer.
 4. The method for fabricating a carbon nanotube aluminum foil electrode according to claim 3, wherein after the Step S1B has a Step S1C: performing a second anodizing process on the aluminum foil, wherein an oxide-removing solution is used to remove the Anodic Aluminum Oxide layer formed in the first anodizing process, and then the aluminum foil is anodized in the oxalic acid solution to form a new Anodic Aluminum Oxide layer on the imprint left by the first anodizing process; the new Anodic Aluminum Oxide layer has a plurality of pores.
 5. The method for fabricating a carbon nanotube aluminum foil electrode according to claim 4, wherein the oxide-removing solution contains 1.8 wt. % chromic acid and 6 wt. % phosphoric acid.
 6. The method for fabricating a carbon nanotube aluminum foil electrode according to claim 4, wherein the first anodizing process and the second anodizing process are undertaken at a voltage of 30-40 V and a temperature of 5-15° C. for 12 minutes, and wherein the oxalic acid solution has a concentration of 0.3 M.
 7. The method for fabricating a carbon nanotube aluminum foil electrode according to claim 4, wherein after the Step S1C has a Step S1D: after the second anodizing process, soaking the aluminum foil in a phosphoric acid solution containing 5 wt. % phosphoric acid at a temperature of 30° C. for 40 minutes to remove a barrier layer that is formed on the bottoms of the pores and contacts the aluminum foil.
 8. The method for fabricating a carbon nanotube aluminum foil electrode according to claim 1, wherein the catalytic material is cobalt, and wherein the electroplating is undertaken in an electroplating solution containing 5 wt. % CoSO₄.7H₂O and 2 wt. % H₃BO₃, at an alternating voltage of 13.6 V and a frequency of 60 Hz, for 40 seconds.
 9. The method for fabricating a carbon nanotube aluminum foil electrode according to claim 1, wherein in Step S3, the carbon nanotubes are grown with a Chemical Vapor Deposition method, and wherein argon is the carrier gas and acetylene is the carbon source, and wherein the growth is undertaken at a temperature of 575-610° C. for 15-90 minutes, and wherein the flow rate of argon is 100 sccm, and the flow rate of acetylene is 50 sccm.
 10. The method for fabricating a carbon nanotube aluminum foil electrode according to claim 1, wherein the anodizing solution is formed via mixing ammonium adipate and deionized water. 